The purpose of the proposed research is to gain insight into the etiology of antisocial behavior (AB; delinquency and conduct disorder symptoms) through study of the mechanisms underlying sex differences in antisociality. Research has repeatedly shown that males engage in more AB than females, and study of the causes of this sex difference can provide clues about the causes of AB within each sex. This conceptual approach to understanding the etiology of AB helps isolate potential risk mechanisms for AB to effects of biological sex and/or psychological gender. An understanding of the mechanisms underlying the sex difference in AB will be achieved via conduct of two separate studies with complementary strengths and weaknesses. The first study will focus on current self-reported delinquency assessed via paper-and-pencil questionnaire surveys in a population-based sample of 85,000 male and female Iowa schoolchildren between 10-19 years of age. The second study will focus on retrospectively-reported lifetime conduct disorder symptoms assessed by telephone interview in a genetically informative sample of 6,225 Australian same-sex male, same-sex female, and opposite-sex twins. Both studies will examine sex differences in vulnerability to risk for AB (the strength of the relation between a specific risk factor for AB and AB is greater for one sex than the other) as well as sex differences in exposure to risk for AB (mean-levels of a risk factor are greater for one sex than the other) in an effort to better understand the causes of the sex difference in the prevalence of AB. Individual (impulsivity), familial (parenting), and extra-familial (neighborhood) risk factors will be examined. Ultimately, findings from the proposed research can be integrated with prior and future research to form a theory of the causes of antisociality that can explain the vast sex difference in the prevalence of AB. Further, findings from the proposed research can inform the understanding of a growing problem in the United States: crime. The cost of crime in the United States, including victims' pain, suffering, and reduced quality of life is $450 billion a year. Such high costs to society demand the attention of the research community to study the biopsychosocial causes of criminal or antisocial behavior and inform policy makers about prevention and intervention strategies. [unreadable] [unreadable] [unreadable]